Devices are known for the creation of an oxygen-free working atmosphere, or more precisely, a working atmosphere as low in oxygen as possible. In melting metal, a nitrogen or argonate atmosphere serves to avoid oxidation or to hold it to a lower limit. Areas that are to be made safe from fire are connected with pressure tanks that contain compressed nitrogen, carbon dioxide or similar inert gases, and their valves open when a certain temperature limit is exceeded. In particularly high-risk areas, halones (halogenated hydrocarbons) are used. Inert gases are also used as propellant and buffer gases in finishing units to avoid oxidation of the finish or at least to minimize it. However, all previously known inert gases are extremely expensive and must therefore be used sparingly.
After continuous application of metal layers on non-metallic workpieces became known through German Patent No. 21 42 474 (Spraying of Molten Metals on e.g., Plastic Surfaces), and it became obvious that such sprayed metal surfaces oxidized seriously during spraying and became porous, like sintering. The need to conduct the spraying process in a low-oxygen working atmosphere arose. Through the use of inert gases (argon and helium), a somewhat more homogeneous metal surface was attained; however the costs of the inert gases prevented sufficient use as the buffer gas surrounding the spraying area.
An invention described in German Published Patent Application No. 32 33 925 attempted to eliminate the defects described in the pioneering invention, German Patent No. 21 42 474, without using expensive inert gases. To prevent oxidation of the sprayed metal during the spraying process, combustion waste gases from the combustion of natural gas were used in a working compartment closed on all sides. The spraying took place in this compartment and the combustion waste gases were used in compressed form in the circulation system as propellant gases for the arc spray guns. Combustion waste gases were thus used as essentially oxygen-free propellant and buffer gases in a spraying process.
Completely aside from the fact that working in the spraying compartment required great effort--the operator stretched his hands into the compartment through sleeves and there controlled the spray gun, and due to the restricted space in the compartment, only smaller workpieces could be processed, in the nature of the device--progress with reference to the quality of the sprayed surface was visible, but not satisfactory. Next to surface sections with fine pores, there were necessarily sections with coarser pores. The results were similar to the results from using inert gases as the propellant, as described above.
In industry, combustion gases were used neither as propellant nor buffer gases, nor in melting metal, nor in the finishing process, certainly for the reasons mentioned above. Because the propellant or buffer gas pressure varied constantly, greater or lesser amounts of outside atmospheric air could penetrate and the results remained uneven. The pressure variations at the spray guns had an effect back on the gas burner and disturbed the combustion process; with larger pressure variations, (i.e., backpressure), the burner might even be extinguished, so that it was possible to create an automatic system only to a limited extent. The use of combustion gases as fire-extinguishing gases thus was precluded from the first.
The present invention starts from the state of the art described in German Published Patent Application No. 32 33 925. The object of this invention is to eliminate the defects that impaired the prior art apparatus and process, as well as the known use of inert gases as propellant and buffer gases. It is a further object of this invention to create a combustion gas as a neutral propellant and buffer gas that is uniform in terms of pressure and composition at the point of application. It is also an object of the invention to provide an apparatus capable of producing such inert gases economically in the desired large quantities, and that could be used anywhere without problems.
These objects of the invention are met by the use of the apparatus and process described below and in the claims.